Face-Shape Secrets May Lie in 'Junk' DNA

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Face shape is largely determined by genetics, yet no two faces
are entirely alike. How do genes bring about faces with subtle
differences while avoiding dramatic disruptions and facial
malformations such as cleft lip and palate? The answer may be in
the "junk DNA," a new study has found.

Noncoding DNA, sometimes
called junk DNA, refers to sequences in a genome that don't
produce proteins, some of which are thought to have no known
biological function.

Studying mice, researchers identified more than 4,000 small
regions in the genome that are likely a type of noncoding DNA
called enhancers, which amplify the expression of a gene. In this
case, these regions were active while the face of a mouse embryo
developed, according to the study, detailed in the Oct. 25 issue
of the journal Science.

Most of these enhancer sequences are found in humans as well, so
it is likely that they have similar face-shaping functions in
humans, the researchers said. [ 5 Face-Shaping Genes Identified ]

"Our results suggest it is likely there are thousands of
enhancers in the human genome that are somehow involved in
craniofacial development," study researcher Axel Visel, a
geneticist at Lawrence Berkeley National Laboratory's genomics
division, said in a statement. "We don't know yet what all of
these enhancers do, but we do know that they are out there and
they are important for craniofacial development."

To test whether these enhancers are indeed important in shaping
the face, the researchers deleted three of the enhancers in
mice and compared them with normal mice at 8 weeks of age. The
results showed that each enhancer deletion caused a distinct set
of differences in the shape of the face — for instance, causing
an increase or decrease in facial length and an increase or
decrease in the width of various parts of the face, such as the
base of the skull or the palate.

In the study, to avoid the challenge of
recognizing individual mouse faces, the researchers created
3D images using a process called microcomputed tomography to link
changes in face shape with alterations in the function of each of
the enhancers.

Identifying enhancers that regulate a gene's activity is
challenging, because such enhancers aren't necessarily located
next to their target gene; rather, they could be acting from
"long-distance" locations in the genome.

Many of the genetic defects that cause facial flaws such as
clefts of the lip or palate have been identified, but only a
small number of genes have been implicated in
normal variation of the face's shape, the researchers
said.

Studying genes that drive normal facial variations would offer an
opportunity for human geneticists to look for mutations
specifically in enhancers that may play a role in birth defects,
Visel said.